Hi Alessio,
You can configure the keyboard layout when using
the VNC protocol via the Console tab of the Edit VM window in the Administration Portal only. See
Accessing_Console_Options for more information..
The VM portal doesn't support that for VNC yet, but we plan to support that on coming releases
Regarding the web based noVNC console, AFAIK it doesn't support keyboard layout very well but worth checking. I also recommend trying both Firefox and Chrome to see which one supports it better.
Regards,
Sharon